Conventionally, in the field of communication, power line communication (PLC) technology is known in which a communication signal in a predetermined frequency band is transmitted superimposed on distributed power using a power line as a transmission line. PLC is attracting attention as technology that enables easy construction of a network because there is no need to provide a dedicated communication line separately from the power line, since the power line for supplying electric power is also used as the transmission line for communication apparatuses to communicate with each other. Note that, in a case where the power line is a power line for AC power distribution, the frequency band of the communication signal is set to a frequency band that is sufficiently higher than the frequency of the AC power that is supplied through the power line.
Incidentally, a power line that is wired to a building or the like normally is not assumed to have a PLC communication signal superimposed thereon, and is simply branched to branch lines from a trunk line using a joint box or the like. Thus, with the PLC communication signal, signal reflection and loss occur at branch points from the trunk line to the branch lines in the power line, possibly leading to a deterioration in communication quality.
As a technique to improve the communication quality of PLC, a technique is proposed in which filter circuits (filter devices) are provided at vicinities (or at branch points) of the branch points from the trunk line to the branch lines (see Patent Document 1: Japanese Patent No. 4284317 and Patent Document 2: Japanese Patent No. 3969451). With the configuration described in Patent Document 1, the filter circuits are provided only at some branch lines to which the PLC communication apparatuses are not connected among the branch lines, and as a result noise and impedance caused by branch lines that are not engaged in communication and electrical apparatuses connected to branch lines that are not engaged in communication are prevented from affecting the PLC communication. The filter device described in Patent Document 2 activates filter processing by a noise filter when an electrical apparatus such as an inverter apparatus or an electric drill is connected to the power line, and as a result noise emitted from the electric apparatuses to the power line is reduced.
However, the noise filters (filter circuits) described in Patent Documents 1 and 2, while being able to reduce reflection of a communication signal at an open end and noise emitted from an electric apparatus, cannot function in branch lines to which PLC communication apparatuses are connected, since these noise filters do not pass the PLC communication signal. Thus, even if these noise filters (filter circuits) are used, reflection or loss of the communication signal at the branch points from the trunk line to the branch lines in the power line cannot be reduced. Note that with the filter circuit described in Patent Document 1, the connection location needs to be changed depending on which branch line the communication apparatus is connected to. However, it is difficult for a general user to take measures to change the connection location of the filter circuit.